[CIR] Upstream AtomicFenceOp

[FantasqueX]

Overview

Upstream handling for __atomic_thread_fence, __atomic_signal_fence functions, including implemetation and lowering of the cir.atomic.fence operation.

Existing incubator tests

clang/test/CIR/CodeGen/atomic-thread-fence.c

[FantasqueX]

Please assign this to me.

[HendrikHuebner]

Hi, there is already a PR open for this: #168346